1. Field of the Invention
This invention relates to mechanisms for opening and closing the shutter of a disc cassette, and more particularly to shutter opening and closing mechanisms adapted to be used with a disc cassette of the type having no biasing force in closing the shutter.
2. Description of the Related Art
Among the magnetic disc cassettes, for example, there is a type in which when not in use, an opening for entrance of a magnetic head is closed by a shutter. And, this shutter may be either biased by a spring or the like in a direction to be closed, or not.
The opening and closing mechanism for the shutter of the latter sort is illustrated in FIGS. 1 to 3. Referring to FIG. 1, a magnetic disc cassette 2 made of plastic contains a magnetic disc 1 having a center hub 3 fixed thereto. The hub 3 is rotatably fitted in round opening portions 5 formed through the upper and lower panels of the cassette 2 at the center of the area thereof. A PG pin 4 for indexing the phase of rotation of the disc 1 is buried in the center hub 3. Square openings 6 are formed in the upper and lower panels of the cassette 2 for a magnetic head to enter.
A shutter 7 of shape conformed to the openings 6 clamps across the thickness of the cassette 2 and is arranged to open and close the openings 6 when it slidingly moves in directions indicated by arrows P and Q respectively. A rectangular hole 7a is provided through the side wall of the shutter 7.
The cassette 2 has a groove 8 formed in that side wall thereof which is in contact with the side wall of the shutter 7, and has location through holes 9 and 10 upon engagement with respective pins of a magnetic recording or reproduction apparatus to be set in a prescribed position.
The shutter 7 also has a pawl 7b formed as extending upwardly of the inside of the side wall at a point ahead of the hole 7a in a direction in which the cassette 2 moves to be loaded on the apparatus.
Meanwhile, in the right hand lower corner as viewed in FIG. 2 within the cassette 2, there is a lock lever 11 made of synthetic resin or the like.
In FIG. 3, the lock lever 11 is fixed at its root to the housing of the cassette 2 by two fastener pins 12 and 13. Its free end 11a extends into a space beyond the outer side of the opening 6 but inside of the side wall of the shutter and has a hook 11c position as the terminal end of closing movement of the pawl 7b. Another arm 11b of short length normally abuts on a stopper 14, thus limiting the counterclockwise movement of the lock lever 11 as viewed in the drawing.
In the magnetic disc apparatus, on the other hand, there is an opening member 15 arranged in alignment with the groove 8 to open the shutter 7 in engagement with a front edge 7c when the cassette 2 is inserted into the apparatus.
Also, a closing lever 16 is positioned in the apparatus in parallel with the path of movement of the cassette 2. This closing lever 16 is fixed at its root to the casing of the apparatus. Its free end 16a is formed to a bent shape and arranged to enter the hole 7a when the front edge 7c comes in contact with the opening member 15.
The operation of the conventional mechanism of such construction is as follows: Before the magnetic disc cassette 2 is loaded, the shutter 7 is in the closed position where the hook 11c of the long arm 11a of the lock lever 11 is in engagement with the pawl 7b. Also, the closing lever 16 urges itself in a direction indicated by arrow H by its resilient property.
With the mechanism in such state, when the magnetic disc cassette 2 is being inserted in a direction indicated by arrow C in FIG. 3, the opening member 15 starts to enter the groove 8 and the bent portion 16a of the closing lever 16 rides on the side wall of the shutter 7.
Then, the opening member 15 starts to contact with the outer edge of the arm 11a of the lock lever 11, and then pushes the arm 11a to a direction of arrow A until the hook 11c disengages from the pawl 7b.
As the cassette 2 is further pushed in, the opening member 15 then contacts with the edge 7c of the shutter 7. Thus, the unlocked shutter 7 is restrained in that position. Therefore, the subsequent movement of the cassette 2 causes the shutter 7 to be opened, with the magnetic disc 1 being exposed through the openings 6. During this time, the bent portion 16a of the closing lever 16 remains in the engaged position in the hole 7a of the shutter 7.
Under this condition, magnetic recording or reproduction is carried out by a magnetic head (not shown).
When the cassette 2 is pulled out of the apparatus, the cassette 2 moves in the reverse direction of that indicated by arrow C. Then, because the bent portion 16a of the closing lever 16 rests in the hole 7a of the shutter, the shutter 7 is left behind in that position, when the cassette 2 is first moved.
Then, as the opening member 15 moves away from the lock lever 11, and the arm 11a returns to the direction of arrow B in FIG. 3, soon after that, the pawl 7b contacts with the tapered portion of the end of the arm 11a, then lifts it in the direction of arrow A, and then engages the hook 11c.
With the shutter 7 fully closed, when the cassette 2 is then further pulled, the closing lever 16 bends outward and removes itself from the hole 7a. Thus, the cassette 2 is expelled from the apparatus under the condition that the shutter 7 is completely closed.
This condition can, however, be insured so long as the load against the sliding movement of the shutter 7 is small enough. As the maximum value of the load can reach about 70 g, if such a large load is put on the shutter, the closing member 16, because its hook 16a being of the bent form, hardly takes stable hold on the edge of the hole 7a of the shutter 7. As a result, the shutter 7 cannot be closed automatically.
If such a problem is solved by increasing the springing force of the closing lever 16, then an alternative problem arises that when the lock lever 11 goes under the bent portion 16a of the closing lever 16 before the bent portion 11a reaches the hole 7a, it is caused to lift up, accidentally releasing the shutter 7 from the locking connection. This allows the strong pressure of the bent portion 16a to start opening of the shutter 7 earlier than the prescribed timing, or exposing the magnetic disc 1 ahead of the prescribed position.
If, in such a state, the cassette 2 is pulled out of the apparatus, the operator will get the cassette 2 with its shutter widely open. Hence, the shutter no longer plays a role, leading to a damage of the magnetic disc 1.
Another problem of the conventional mechanism described above is that because the opening member is provided in separation from the closing member, surplus parts become necessary. By this amount, the number of assembling steps is increased to increase the production cost.